10 years old or 10 year old?
correct grammar form is 10 years old, but the another version is also usually usable.
May be it can be used without "s" when it is some kind of adjective? my 10-year-old sister? Is there any rule?

Or, for example - port label. Is it 20 years old tawny port? or 20 year old tawny port?

Question

10 years old or 10 year old?
correct grammar form is 10 years old, but the another version is also usually usable. 
May be it can be used without "s" when it is some kind of adjective? my 10-year-old sister? Is there any rule?

Or, for example - port label. Is it 20 years old tawny port? or 20 year old tawny port?

Answer

Ten years old comes after a verb (be/become). "She will become ten years old tomorrow. he is ten years old."
Ten year old comes after an article(a,an,the) or adjective(kind,little,etc.). "I am a ten year old boy. That cute, little ten year old gave me some flowers."

Answer

The rule is simple:

My sister is 10 years old.
I have a ten year old sister.

That is the rule.

Answer

Елиот правильно объяснил.

Answer

it should be"10 years old"or"10-year-old"

Answer

Ten years is correct.